Precise wafer temperature control is vital during all thermally activated steps of the integrated circuit fabrication process. These processes include epitaxial and oxide growth, and dielectric and metal film depositions. The trend towards single wafer processing rather than batch processing requires accurate wafer temperature measurement for successful temperature control. Presently, one of the major impediments for temperature control is the lack of a direct and non-contact measurement of the surface temperature of silicon wafers during processing.
Thermocouples are the most common form of temperature monitoring devices in prior processing equipment. Thermocouples have several disadvantages; chief among them is that they contact the wafer. Poor contact causes a loss of accuracy, and contact with the wafer front side damages the wafer, or perturbs the process. Another problem is the lag between the measured temperature and the actual temperature due to the thermal mass of the thermocouple. The thermal mass causes the temperature at the contact point to differ from the true wafer temperature during temperature transients. There are also problems associated with the presence of the metal thermocouple in the harsh environment of the processing chamber. Frequently, the thermocouples are attacked by the process gases, and their output is perturbed by RF voltages during plasma processes.
Pyrometry, which offers a direct, non-contact measurement of temperature, also has several disadvantages. The chief disadvantage of pyrometry is its reliance upon the emissivity of silicon. The emissivity of silicon is a strong function of temperature in the temperature domain of interest. This drawback theoretically could be overcome with an independent measurement of the emissivity, but this has proven difficult to do.
There have been several attempts to use ellipsometry to measure the temperature of silicon substrates. While these techniques are direct and non-contacting, they use expensive ellipsometers with moving parts, and rely on the temperature dependence of n, the real part of the refractive index, to determine the temperature. Since k, the imaginary part of the refractive index, rather than n is more sensitive to wafer temperature, it is preferable to measure k.
Therefore, a need has arisen for a temperature measurement technique which is accurate, which does not interact with wafer processes nor contacts the wafer, and which is inexpensive.